'iPhone slow' explained: The truth about iOS updates

The charge that iOS updates deliberately slow down older iPhones gets regurgitated every so often. That's not a huge surprise. It's sensational so it gets sensationalized. Recently some research into correlation vs. causation in big data analytics and how it shouldn't be misinterpreted was wait for it widely misinterpreted. Whether regurgitators failed to read all the way through the research, or simply decided to ignore it in service of sensationalism doesn't really matter. The results are the same. The misinformation is the same. So, what's really going on with iOS updates and older devices?